1. Field of the Invention
The present invention relates to a soluble polythiophene derivative containing highly coplanar repeating units, and in particular relates to using the soluble thiophene derivative in optoelectronic devices.
2. Description of the Related Art
Recently, due to the fundamental semiconductor and optoelectronic properties of polymer semiconducting material, the material has attracted considerable research interest because of the potential for being used in optoelectronic devices such as organic thin film transistors (OTFTs), organic light-emitting diodes (OLEDs), and organic solar cells (OSCs) and being mass produced. In addition, with the active layer of the optoelectronic device being fabricated by printing technology, material having polymer properties can produce a flexible optoelectronic device with advantages of light-weight, low cost and large area fabrication.
The key development for polymer semiconducting material applicability is based on the soluble conjugated polymer. For the soluble conjugated polymer, a main chain is connected by a conjugated backbone, and the function of a side chain is to increase solubility. However, a main drawback for conjugated polymer applicability in optoelectronic devices is its low carrier mobility, consequently limiting the practicability of optoelectronic devices made with polymer semiconducting material. Therefore, one solution is to synthesize conjugated polymer with high carrier mobility materials to improve the efficiency of optoelectronic devices made from polymer semiconducting material.
For application of polymer semiconducting material by organic thin film transistors (OTFT), polythiophene with carrier mobility of 10−5 cm2/Vs is first used as an active layer. Next, poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorenecobithiophene) (F8T2) are sequentially synthesized, thus improving the carrier mobility of the OTFT device to 10−1˜10−3 cm2/Vs.
For application of polymer semiconducting material by organic solar cells (OSCs), the active layer must be made of high mobility materials that can effectively transport holes and electrons, respectively. Otherwise, if the electrons and holes recombine, the carrier function will disappear, thus reducing power conversion efficiency of the OSCs. In 1979, a solution having hetero-junction structure including an electron donor and an electron acceptor was proposed, which sparked development of OSCs. At the time, initial power conversion efficiency was about 1%. Afterwards, various polymer materials were synthesized to be used as electron donors material and C60 material was used as an electron acceptor material, such as poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with C60, P3HT with PCBM, thus improving power conversion efficiency in the range of 4 to 5%. In 2006, D. Muhlbacher at al. proposed using PCPDTBT (poly[2,6]-(4,4-bis-(2-ethylhexyl)-4H-cyclopental[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) polymer with a low bandgap, which improved power conversion efficiency of the polymer with PC71BM to 3.2%. (For further detail see D. Muhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana, C. Brabec, “High photovoltaic performance of a low-bandgap polymer”, Adv. Mater, 18, pp. 2884-2889 (2006))
The above-mentioned polymers such as P3HT F8T2 and PCPDTBT are polymerized by thiophene derivatives. For the polymers, the main chain is based on a planar conjugated thiophene derivative. Because of interaction between the side-chain molecules, the intermolecules π-π interaction and carrier mobility are improved.
In 2006, Ken-Tsung Wong. at al. synthesized TPT (thiophene-phenylene-thiophene) and applied the material as a light-emitting material. The TPT compound was almost coplanar, so it exhibited good intermolecules π-π interaction. (For further detail see K.-T Wong, T.-C. Chao, L.-C. Chi, Y.-Y. Chu, A. Balaiah, S.-F. Chiu, Y.-H. Liu, Y. Wang, “Syntheses and structures of novel heteroarene-fused coplanar π-conjugated chromophores”, Org. Lett. 8, pp. 5033-5036 (2006))
From the above description, if coplanar conjugated polymer is synthesized, not only does intermolecules π-π interaction improve, but also carrier mobility will be increased.